U.S. Pat. No. 4,040,098, which issued Aug. 2, 1977 to Gary H. Beeson et al, and which is assigned to the present assignee, discloses a video tape editing control system which has particular utility in conjunction with cassette-type helical scan video tape recorders. The video tape in such a video tape recorder is provided with a control track in which magnetic pulses are recorded, one for each frame of video information on the tape. The editing control system described in the patent utilizes the control pulses from the control track of the tape to control the operation of the video tape recorder during the editing procedures, thereby obviating the need for utilizing the complex time codes recorded on the tape and the complicated ancillary equipment which responds to such codes.
The magnetic pulses recorded on the control track of the video tape are detected in the video tape recorder by a magnetic head which produces electrical pulses corresponding to the recorded magnetic pulses, as the video tape is drawn across the magnetic head. The signal level of the electrical pulses from the magnetic head is dependent upon and proportional to the speed at which the video tape is drawn across the head. As the tape is slowed, for example, to below one-half normal play speed, the signal level of the pulses produced by the head decreases rapidly and soon becomes too low to be utilized reliably, and at very low speeds the pulses disappear completely. Therefore, as explained above, the control track on the video tape is ineffective for supplying control pulses to the editing control system at low speeds of the tape, and at a point in the editing process when precise indications of the tape field and of the tape direction are required. The guard band detection system of the invention was conceived to provide a means for enabling the editing control system to determine tape frame positions and tape direction at such low speeds.
In the helical scan video tape recorder the video signals are recorded on the video tape by a pair of video record/playback heads. The heads are mounted on a scanner which moves the heads across the tape even when the tape is stationary, so that the signal output level of the heads is maintained independent of tape speed. The video information recorded on the video tape by these heads takes the form of parallel tracks which extend diagonally across a center portion of the tape. Each diagonal track represents one field of video information, and each pair of the video tracks represents one frame of video information. The individual diagonal video tracks are separated from one another by a diagonal space which bears no recording and which is known as a guard band.
So long as the video tape is being moved at normal play speed during a playback operation, the video record/playback heads are controlled so that each head scans exactly along a previously recorded diagonal video track and does not enter the adjacent guard band. However, when the tape is stopped, or is moving at a slow speed, the video record/playback heads move into the adjacent guard band during each scanning cycle, and thereby create guard band interference in the reproduced video signal. This interference takes the form of a signal burst which occurs in each video field, and these signal bursts are detected by the system of the present invention and are used to provide control pulses when the video tape is moving at slow speeds, and which replace the usual control track pulses which are no longer available for frame counting purposes.
In addition to losing the control track pulses at slow tape speeds, there is the problem of sensing the direction of movement of the tape during such slow speeds, and particularly when the tape is being slowed down to a stop, or started up from a stop. In either situation the control track pulses are lost and the logic circuitry of the editing control system has no way of determining tape direction. The control pulses derived from the guard band interference by the system of the invention are also used to provide an indication of tape direction.
During the recording operation of the video tape recorder, the video information is recorded by each record/playback head at normal speed of the tape, which is 33/4 inches per second in most cases for the 3/4 inch cassette video tape recorders. At normal speed the tape is moving very slowly but the record/playback heads which are located on the scanner rapidly move at a high speed relative to the tape to provide a bandwidth sufficient to record the video signal, sufficient tape being available in each diagonal track to record one full video field of information.
Since two video fields are interlaced to provide one frame of video information, the two fields are recorded in sequence, one by each of the two video record/playback heads as the tape moves across the face of the scanner in the video tape recorder. The helical tracking of the heads on the tape is obtained by having the heads rotate in a plane which is at a slight angle to the direction of tape motion. Thus, a recording length of about seven inches is achieved for each field of video information, on a tape which is actually only 3/4 of an inch wide. Not only is the tape only 3/4 of an inch wide, but only a central portion of the tape is used, because there are two audio tracks along one edge of the tape and a control track along the other edge.
The separation of the tracks of video information after the video signal has been recorded on the tape, as noted above, is called the guard band. This separation is created because the tape is moving as each of the two record/playback heads are scanned across the tape. Therefore, as one head leaves the tape from one edge, after finishing its recording of its field, the other head enters the tape from the other edge to begin recording the next field. Since the tape has moved from the position that it was in when the recording of the previous field as initiated, the second field is recorded along a second diagonal track separated from the previous track by the guard band.
Since the video information is recorded at a specific speed, it follows that when the tape is moving at any speed other than the specific speed during the playback operation of the video tape record, when the video information is being read from the tape, the effective angle of scanning of the record/playback heads changes. When the video tape recorder is playing back the video information on the tape during normal play speed of the tape, the electronics of the video tape recorder synchronizes the position of the heads as they enter the tape so that the diagonal tracks representing the fields recorded on the tape will line up precisely with the entry of the heads into the scanning area.
However, if the tape slowed to below normal play speed, the scanner will continue to turn at a constant speed, but the distance the tape moves between successive scans becomes less and less. This results in an effective angle change of the scanning process. As the scanning angle changes, the heads will no longer follow the recorded tracks precisely. For example, if by chance one of the record/playback heads enters the scanning area of the tape accurately centered on a track during a playback operation at less than playback speed, then, since the head is travelling at a different effective angle than was travelled when the field of video information was recorded, it will be precisely centered on that track only for a short distance and it will gradually move off the track and into the guard band between the track and the adjacent track.
The distance the head moves off the center line of the track, will vary with the speed of the tape. The zero variation, of course, being when the tape is running at play speed, and the maximum variation being when the tape is running in the reverse direction at play speed. Thus, at play speed the head and the track are in perfect alignment, and as tape speed becomes slower and slower, the variation becomes more and more pronounced. At zero tape speed, the head will move across the guard band area and will enter the adjacent track.
When the digression of either one of the record/playback heads into the guard band from a first track begins, there will be no apparent degradation of the signal as long as the trailing side of the head is still on the first track and the leading side of the head is still in the guard band area. But as soon as the leading edge of the head begins to impinge upon the adjacent track, the head will begin to pick up composite video signals from that track as well as composite video signals from the first track, and this creates a disturbance in the output signal of the head which will be referred to herein as guard band interference signals. The amplitude of the guard band interference signals increase as the head progresses until a point is reached where equal video information from the two adjacent tracks is being read by the head. As the head continues to move away from the first track and toward the adjacent track, the interference then drops off until the head is no longer reading the original track, but is reading video information solely from the adjacent track.
The guard band interference signals can be observed on an oscilloscope, and they are represented by an extreme amplitude change and a large amount of interference and noise in the composite video signal. The guard band interference signals also can be observed on a television monitor as a band of distorted video information. The band observed on the television monitor is relatively narrow, typically 10-20% of the height of the frame as displayed on the monitor. Therefore the entire picture is not lost.
Probably, the easiest way to begin to understand how the guard band interference signals may be used for counting tape frames and for designating tape direction at low tape speeds is by considering the tape in a stopped condition. If the tape is stopped, and a particular record/playback head happens to enter the video recording area precisely centered upon one track of video recording, it will exit the area precisely centered on the adjacent track. This causes the distorted band to appear on the television monitor midway in the field interval, or half-way between the top and bottom of the monitor screen. Any motion of the tape in either direction, will cause the band to move either up or down the screen. If, on the other hand, a particular record/playback head entered the tape centered on the guard band between two adjacent tracks, then the band appears at the extreme top edge of the screen of the video monitor and possibly again at the bottom edge of the screen.
As the tape begins to move slowly, the entry position of the head relative to the center of the track will change. That is, the head will gradually move toward the adjacent track causing the distortion band to move its position as observed on the video monitor. As tape moves in one direction very slowly, the band will appear to move from the bottom to the top of the screen, and will exit from the top of the screen and reappear at the bottom. As the tape moves in the opposite direction, the band will move from the top of the screen towards the bottom, and will exit from the bottom and re-appear at the top of the screen.
The motion of the distortion band, therefore, is in a predictable direction, and indicates a known amount of tape movement as the band progresses across the monitor screen. For example, if the band progresses from the top to the bottom of the screen, its second exit from the bottom of the screen will indicate that the tape has progressed one frame distance, or 1/16th of an inch. These progressions of the distortion band are utilized by the system of the invention to measure tape motion. The reason this can be done is because, as explained above, the scanner is running continuously, whether the tape is moving or not, so that the band is always available. If the position of the band remains static, on the screen of the monitor, it simply means that the tape is not moving. As the tape begins to move, the band will progress either toward the top of the screen and disappear, to reappear at the bottom; or it will progress towards the bottom of the screen and re-appear at the top, depending upon the direction of tape motion.